UNIVERSITY   OF  CALIFORNIA   PUBLICATIONS 

COLLEGE  OF  AGRICULTURE 

AGRICULTURAL  EXPERIMENT  STATION 

BERKELEY,  CALIFORNIA 


CONTROL  OF  GROUND   SQUIRRELS 
BY  THE  FUMIGATION  METHOD 


BY 
G.  R.  STEWART  AND  JOHN  S.  BURD 


BULLETIN  No.  302 

December,   1918 


UNIVERSITY    OF  CALIFORNIA   PRESS 

BERKELEY 

1918 


Benjamin  Ide  Wheeler,  President  of  the  University. 

EXPERIMENT  STATION  STAFF 

HEADS    OF   DIVISIONS 

Thomas  Forsyth  Hunt,  Director. 

Edward  J.  Wickson,  Horticulture  (Emeritus). 

Herbert  J.  Webber,  Director  Citrus  Experiment  Station;  Plant  Breeding. 

Hubert  E.  Van  Norman,  Vice-Director;  Dairy  Management. 

William  A.  Setchell,  Botany. 

Myer  E.  Jaffa,  Nutrition. 

Charles  W.  Woodworth,  Entomology. 

Ralph  E.  Smith,  Plant  Pathology. 

J.  Eliot  Coit,  Citriculture. 

John  W.  Gilmore,  Agronomy. 

Charles  F.  Shaw,  Soil  Technology. 

John  W.  Gregg,  Landscape  Gardening  and  Floriculture. 

Frederic  T.  Bioletti,  Viticulture  and  Enology. 

Warren  T.  Clarke,  Agricultural  Extension. 

John  S.  Burd,  Agricultural  Chemistry. 

Charles  B.  Lipman,  Soil  Chemistry  and  Bacteriology. 
t  Clarence  M.  Haring,  Veterinary  Science  and  Bacteriology. 

Ernest  B.  Babcock,  Genetics. 

Gordon  H.  True,  Animal  Husbandry. 

James  T.  Barrett,  Plant  Pathology. 

Fritz  W.  Woll,  Animal  Nutrition. 

Walter  Mulford,  Forestry. 

W.  P.  Kelley,  Agricultural  Chemistry. 

H.  J.  Quayle,  Entomology. 

J.  B.  Davidson,  Agricultural  Engineering. 

Elwood  Mead,  Rural  Institutions. 

H.  S.  Reed,  Plant  Physiology. 

James  C.  Whitten,  Pomology. 
fFRANK  Adams,  Irrigation  Investigations. 

C.  L.  Roadhouse,  Dairy  Industry. 

Frederick  L.  Griffin,  Agricultural  Education. 

John  E.  Dougherty,  Poultry  Husbandry. 

S.  S.  Rogers,  Olericulture. 

R.  S.  Vaile,  Orchard  Management. 

J.  G.  Moodey,  Assistant  to  the  Director. 

Mrs.  D.  L.  Bunnell,  Librarian. 

DIVISION  OF  AGRICULTURAL  CHEMISTRY 

John  S.  Burd.  Paul  L.  Hibbard. 

{Guy  R.  Stewart.  Walter  H.  Dore. 

Dennis  R.  Hoagland.  t  Arthur  W.  Christie. 

James  C.  Martin. 


t  In  military  service. 

f  In  co-operation  with  office  of  Public  Roads  and  Rural  Engineering,  U.   S. 
Department  of  Agriculture. 


CONTROL  OF  GROUND  SQUIRRELS  BY  THE 
FUMIGATION   METHOD 

By  G.  B.  STEWART  and  JOHN  S.  BURD 


In  the  present  war  emergency  the  stimulation  of  food  production 
has  become  the  active  aim  of  everyone  connected  with  agriculture. 
It  is  evident  that  anything  which  prevents  loss  of  food  after  it  is 
raised  is  just  as  important  as  an  increase  in  the  annual  crop.  Prob- 
ably the  greatest  preventable  loss  of  foodstuffs  in  the  State  of  Cali- 
fornia is  due  to  the  depredations  of  the  common  ground  squirrel. 
This  animal  is  estimated  by  various  authorities1  to  consume  from 
five  to  twenty  million  dollars  worth  of  food  and  forage  annually  in 
California.  In  some  of  the  larger  single  counties  the  loss  is  said  to 
reach  five  hundred  thousand  dollars  yearly.  This  explains  the  present 
active  interest  in  ground-squirrel  control.  The  present  investigation 
was  undertaken  to  aid  this  emergency  work  and  to  assist  in  the  cam- 
paigns carried  on  by  the  Horticultural  Commissioners  and  by  the 
Farm  Bureaus  of  the  state. 

The  five  most  important  methods  of  ground-squirrel  control,  as 
pointed  out  by  Dixon  in  Circular  181  of  this  station,  are :  "  (1)  poison- 
ing with  strychnine;  (2)  fumigation  with  carbon  bisulfide;  (3)  trap- 
ping; (4)  shooting;  (5)  encouragement  of  the  natural  enemies  of  the 
ground  squirrel."  The  first  of  these  methods,  poisoning  with  strych- 
nine-coated barley,  is  undoubtedly  the  most  satisfactory  and  the  least 
expensive  to  use  for  a  preliminary  treatment.  The  work  of  the  United 
States  Biological  Survey,  the  United  States  Public  Health  Service, 
and  of  several  state  experiment  stations,  has  shown  that  the  intelligent 
distribution  of  poisoned  barley  will  remove  approximately  ninety  per 
cent  of  the  squirrel  population.  Reducing  the  remaining  ten  per 
cent  should  be  the  constant  aim  of  effective  squirrel  control.  The 
females  of  this  small  fraction  will  produce  an  average  of  eight  young 
in  a  litter.  If  these  young  are  added  to  the  mature  survivors  there 
will  be  at  least  half  as  many  squirrels  to  eat  the  crop  as  were  present 
before  the  poisoning  started.  It  can  be  readily  seen  that  a  follow-up 
treatment  is  essential  after  each  poisoning  campaign.  The  aim  of 
squirrel  extermination  should  be  to  remove  all  possible  survivors 
before  the  breeding  season.  One  female  killed  in  March  may  remove 
as  many  as  nine  from  the  grain  fields  the  next  July. 


208  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

The  best  method  which  has  so  far  been  devised  for  removing  the 
remaining  squirrels  is  fumigation  with  carbon  bisulfide.  This  does 
not  depend  on  the  co-operation  of  the  squirrels  as  does  poisoning. 
When  green  food  is  abundant  it  has  been  noticed  that  many  squirrels 
will  not  eat  the  poisoned  grain.  Others  are  especially  strong  and 
vigorous  and  so  escape  the  action  of  the  poison.  With  carbon  bisul- 
fide all  that  is  necessary  is  to  bring  the  squirrel  into  an  atmosphere 
which  contains  approximately  2%  of  the  gas  and  keep  it  there  for  at 
least  twenty  minutes. 

Carbon  bisulfide  is  best  used  when  the  ground  is  moist.  The  soil 
does  not  then  absorb  the  gas  in  the  open  pore  spaces  and  the  cracks  are 
closed  through  which  it  might  escape.  If  carbon  bisulfide  is  used  at 
other  seasons  of  the  year  either  larger  quantities  must  be  employed  or 
special  apparatus  must  be  used  which  will  pump  the  gas  through  the 
burrow  with  sufficient  speed  to  send  it  to  the  nest  chamber  before  it 
has  a  chance  to  be  absorbed  by  the  soil. 

CARBON  BISULFIDE   AS  A  FUMIGANT 

The  first  person  to  use  carbon  bisulfide  as  a  method  of  squirrel 
control  was  Dr.  E.  W.  Hilgard,  for  many  years  director  of  this  station. 
In  18762  he  advised  its  use  upon  squirrels  because  he  had  noted  it  was 
successfully  used  in  France  to  exterminate  rats  in  sewers.  Two  years 
later3  he  described  his  work  on  the  university  campus,  and  the  success- 
ful manner  in  which  he  had  removed  the  greater  part  of  the  squirrels 
from  the  gardens. 

Since  that  time  three  general  methods  of  use  have  been  developed. 
The  first  is  that  originally  recommended  by  Dr.  Hilgard.  One  to 
two  ounces  of  carbon  bisulfide  are  poured  on  cotton  waste  and  placed 
deep  down  in  one  or  two  of  the  most  used  entrances  of  each  burrow ; 
all  holes  are  closed  with  clods  of  earth  or  sod.  These  are  inspected 
the  following  day  to  observe  whether  the  treatment  has  been  effective. 
The  few  which  have  been  opened  by  the  squirrel  are  re-treated  with  a 
slightly  larger  dose  and  inspected  later.  This  is  commonly  termed 
"waste  ball"  method  of  treatment. 

Another  very  effective  procedure  has  been  developed  by  the  United 
States  Public  Health  Service.  In  it  a  machine  called  a  "destructor"4 
is  employed  to  drive  the  carbon  bisulfide  gas  into  the  further  ends  of 
the  burrow.  The  detailed  plan  of  this  instrument  was  reproduced 
in  Circular  181  of  this  station.  Briefly,  it  consists  of  a  pump  of  large 
bore  which  forces  a  strong  current  of  air  through  a  chamber  in  which 
a  measured  quantity  of  carbon  bisulfide  is  vaporized.  The  outlet  hose 
of  the  machine  is  thrust  down  one  of  the  principal  entrances  of  the 


CONTROL  OF  GROUND  SQUIRRELS  BY  THE  FUMIGATION  METHOD       209 

squirrel  burrow,  the  soil  is  filled  in  around  it  and  all  other  entrances 
are  closed.  About  fifteen  double  strokes  of  the  pump  are  sufficient 
to  fill  the  burrow  with  a  deadly  quantity  of  gas.  One  or  two  cheaper 
machines  have  been  devised,  but  so  far  as  the  writers  have  been  able  to 
learn,  neither  these  nor  the  "destructor"  are  at  present  on  the  market. 
This  is  due  partly  to  the  present  high  cost  of  the  materials  employed 
and  partly  to  a  lack  of  appreciation  of  the  greater  efficiency  which  is 
achieved  by  the  use  of  a  pump  machine. 

The  other  variation  in  the  use  of  carbon  bisulfide  consists  in  explod- 
ing the  gas  in  the  burrow.  Carbon  bisulfide  is  extremely  inflammable 
and  very  explosive.  The  common  procedure  is  to  place  the  waste  ball 
saturated  with  carbon  bisulfide  in  the  burrow,  close  the  entrance  with 
a  loose  clod  of  earth  for  two  or  three  minutes  to  allow  the  gas  to 
vaporize,  then  to  remove  the  clod  and  stepping  back  and  to  one  side 
to  apply  a  small  lighted  torch  consisting  of  a  cloth  soaked  in  kerosene 
wrapped  around  a  thin  piece  of  pipe.  The  explosion  will  be  extremely 
vigorous  and  has  the  advantage  of  indicating  whether  all  the  entrances 
of  the  burrow  have  been  previously  closed.  This  method  of  operation 
should  never  be  employed  when  the  grass  is  at  all  dry.  Many  serious 
fires  have  been  caused  in  this  way  during  the  drier  periods  of  the  year. 

All  three  methods  of  fumigation  have  had  their  advocates  in  the 
past.  There  is  good  evidence  from  the  figures  of  Surgeon  Long4  that 
the  treatment  by  machines  of  the  destructor  type  is  the  most  eco- 
nomical. He  found  that  200  to  250  holes  per  gallon  could  be  treated 
with  the  destructor,  against  50  to  60  holes  by  the  waste  ball  method. 

The  question  of  the  relative  effectiveness  of  the  exploded  as  com- 
pared to  the  unexploded  gas  depends  upon  two  factors,  viz.,  whether 
the  exploded  or  the  unexploded  gas  is  the  more  poisonous,  and  the 
completeness  with  which  either  gas  is  distributed  into  the  more  dis- 
tant parts  of  the  burrow.  If  the  unexploded  and  exploded  gas  are 
equally  poisonous  the  more  effective  of  the  two  methods  is  the  one 
which  will  distribute  the  gas  the  more  quickly  and  thoroughly.  If 
the  unexploded  and  exploded  gas  are  not  equally  poisonous  the  more 
poisonous  form  may  be  the  more  effective  even  if  its  dissemination  is 
slower.  A  -knowledge  of  the  kinds  and  amounts  of  gas  resulting  from 
the  explosion  of  carbon  bisulfide  and  a  comparative  study  of  their 
toxicity  to  squirrels  is  therefore  an  important  part  of  the  present 
investigation. 

It  has  been  generally  assumed  that  the  products  of  the  explosion 
of  carbon  bisulfide  in  an  excess  of  air  consist  exclusively  of  carbon 
dioxide  and  sulfur  dioxide.  This  assumption  is  not  justified,  because 
the  course  of  the  reaction  may  be  modified  by  several  factors.     In 


210  UNIVERSITY    OF    CALIFORNIA — EXPERIMENT    STATION 

the  field  the  conditions  which  are  most  likely  to  affect  the  explosion 
and  change  the  products  of  combusion  are,  the  humidity  of  the  air, 
the  temperature,  and  the  atmospheric  pressure.  It  is  also  possible 
that  the  products  formed  from  gaseous  carbon  bisulfide  will  be 
different  from  those  produced  from  burning  the  liquid  material. 

We  have  found  in  laboratory  experiments  reported  elsewhere5  that 
where  carbon  bisulfide  is  completely  vaporized  and  exploded  in  the 
presence  of  an  excess  of  air,  the  largest  portion  of  the  gas  (40%  to 
60%)  went  to  form  carbon  dioxide  and  sulfur  dioxide,  from  25  to 
35%  formed  carbon  monoxide  and  sulfur  dioxide,  and  15  to  30% 
remained  unexploded. 

The  carbon  dioxide  formed  is  well  known  to  be  non-poisonous. 
It  is  the  gas  used  to  carbonate  soda  water  and  similar  beverages. 
The  only  way  that  it  could  cause  death  would  be  if  it  were  present 
in  such  large  amount  that  it  excluded  the  air.  The  squirrel  would 
then  die  from  suffocation ;  this  would  require  a  vastly  greater  quantity 
than  is  ever  present  from  the  explosion  in  the  burrow. 

Carbon  monoxide,  on  the  contrary,  is  poisonous  to  both  animals 
and  human  beings  if  as  much  as  one-tenth  of  one  per  cent  is  present 
in  the  air.  This  carbon  monoxide  is  the  principal,  cause  of  the 
poisonous  effects  from  illuminating  gas  in  which  it  may  be  present 
in  amounts  of  two  to  twenty  per  cent. 

The  effect  of  the  sulfur  dioxide  is  less  well  known.  The  Selby 
Smelter  Commission6  found  five-hundredth s  of  one  per  cent  of  sulfur 
dioxide  caused  a  sense  of  acute  suffocation  to  human  beings,  but  no 
exact  information  has  been  found  as  to  its  effect  on  squirrels. 

EXPERIMENTS  WITH  SQUIRRELS 
In  the  earlier  of  these  experiments  an  exact  mixture  of  carbon 
bisulfide  and  air  was  made  up  in  a  series  of  communicating  carboys. 
The  outlet  of  these  carboys  was  connected  with  the  lower  opening  of 
a  glass  bell  jar  of  sufficient  size  to  contain  a  wire  cage  in  which  the 
squirrel  was  placed.  A  gentle  stream  of  water  was  then  passed  into 
the  farthest  carboy  which  caused  a  steady  current  of  gas  to  flow 
through  the  bell  jar.  In  this  manner  the  squirrel,  kept  in  an  atmos- 
phere of  constant  composition,  was  at  all  times  under  observation. 
Only  a  portion  of  the  gas  contained  in  the  system  was  used  in  any 
set  of  experiments.  The  composition  of  the  part  which  the  squirrel 
breathed  therefore  could  not  be  changed  by  coming  in  contact  with 
the  water.  The  plan  of  this  apparatus  is  indicated  in  Figure  1.  In 
this  manner  squirrels  were  treated  with  air  containing  4%,  2%,  and 
1%  carbon  bisulfide. 


CONTROL  OF  GROUND  SQUIRRELS  BY  THE  FUMIGATION  METHOD       211 


i 

I— 4 

2, 
O 

1—4 

3 
o 


g 


C7i 


(W 


P3 


O)-* 


212 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


The  detailed  results  of  the  treatments  are  given  in  Table  I.  It 
was  found  that  4%  gas  did  not  kill  the  squirrels  more  quickly  than 
2%,  while  1%  was  too  weak  to  be  fatal  within  less  than  an  hour  and 
a  half.  With  2%  gas  the  squirrels  partially  collapsed  in  from  six 
to  ten  minutes  and  in  seventeen  to  twenty  minutes  had  ceased 
breathing. 


TABLE  I. 
EXPERIMENTS   WITH    VARYING   CONCENTRATIONS   OF    CARBON 

BISULPHIDE 

Percentage  Manner  of  use 

of  gas 

4%  Unexploded,  constant  current  of  gas 

4%  Unexploded,  constant  current  of  gas 

4%  Unexploded,  constant  current  of  gas 

2%  Unexploded,  constant  current  of  gas 

2%  Unexploded,  constant  current  of  gas 

2%  Unexploded,  caged,  artificial  burrow 

2%  Unexploded,  caged,  artificial  burrow 

2%  Unexploded,  caged,  artificial  burrow 

1%  Unexploded,  constant  current  of  gas 

1  %  Unexploded,  constant  current  of  gas 

4%  Exploded,  caged,  artificial  burrow 

2%  Exploded,  caged,  artificial  burrow         female        pregnant        15  min. 


Sex 

Weight 

Time  of 
Treatment 

Result 

female 

22  min. 

death 

male 

537  g. 

13  min. 

recovered 

male 

575  g. 

14  min. 

recovered 

male 

651  g. 

21  min. 

death 

male 

633  g. 

\llA  min. 

death 

female 

small 

20  min. 

death 

male 

large 

20  min. 

death 

female 

small 

21  min. 

death 

male 

537  g. 

1  hr. 

recovered 

male 

575  g. 

l^hrs. 

death 

female 

small 

16  min. 

death 

2%     Exploded,  caged,  artificial  burrow 


male 


old,  large       20  min. 


2%     Exploded,  caged,  artificial  burrow  female       small 


23  min. 


2%     Exploded,  caged,  artificial  burrow         female   large,  strong     39  min. 


death 
death 
death 
death 


An  artificial  burrow  was  next  constructed  from  5-foot  lengths  of 
six-inch  iron  soil  pipe.  By  joining  several  of  these  together  with 
wet  clay  it  was  possible  to  modify  the  length  and  volume  of  the  bur- 
row and  also  change  its  elevation  when  desired.  A  length  of  eleven 
feet  was  first  employed.  The  upper  end  was  closed  by  a  tight  wooden 
cover  firmly  wired  on  and  plastered  with  clay  to  prevent  escape  of 
gas.  In  this  wooden  cover  an  inch  hole  was  bored  and  closed  with  a 
rubber  stopper.  Through  this  hole  measured  amounts  of  carbon 
bisulfide  could  be  added  and  on  vaporizing  could  readily  flow  toward 
the  other  end  of  the  pipe  which  was  placed  a  few  inches  lower.  The 
squirrel  was  placed  in  a  large  glass  specimen  jar,  the  upper  end  of 
which  was  covered  with  wire  screen.  The  mouth  of  this  jar  was 
shoved  into  the  expanded  end  of  the  pipe  and  the  space  between  the 
neck  of  the  jar  and  the  pipe  packed  with  clay  The  squirrel  remained 
in  plain  view  from  the  outside  and  could  not  escape  into  the  pipe,  but 
still  received  gas  just  as  readily  as  if  he  had  been  within  the  pipe. 
The  form  of  the  artificial  burrow  is  shown  in  Figure  2. 


CONTROL  OF  GROUND  SQUIRRELS  BY  THE  FUMIGATION  METHOD       213 

A  number  of  squirrels  were  treated  with  carbon  bisulfide  in  this 
artificial  burrow  and  succumbed  in  approximately  the  same  time  with 
both  2%  and  4%  gas  as  did  those  which  had  been  placed  in  a  stream 
of  gas  in  the  glass  bell- jar. 

A  second  group  of  squirrels  were  now  treated  in  this  artificial 
burrow  with  4%  and  2%  gas  which  had  been  exploded.  The  carbon 
bisulfide  was  added  through  the  hole  in  the  cover  at  the  upper  end 
of  the  burrow  and  three  minutes  was  allowed  to  elapse  in  order  to 
give  the  liquid  time  to  vaporize.  A  lighted  match  was  then  thrust 
in  the  hole  and  a  vigorous  explosion  followed.  The  white  smoke 
resulting  from  the  explosion  was  driven  through  the  pipe  and  into  the 
glass  jar  containing  the  squirrel.  It  was  evident  that  the  explosion  had 
some  slight  effect  in  distributing  the  gas  more  thoroughly  throughout 
the  pipe.  None  of  the  squirrels  in  any  of  the  experiments  appeared 
to  be  at  all  harmed  by  the  explosion.  They  were  clearly  frightened  but 
did  not  appear  to  be  injured  in  any  way  by  the  concussion  or  by  the 
flame  which  generally  ran  through  the  pipe.  The  squirrels  which  were 
treated  with  the  exploded  gas  showed  a  little  greater  variation  in  the 
effect  which  the  gas  had  upon  them  than  had  been  the  case  with  the 
unexploded  carbon  bisulfide.  In  general,  however,  they  succumbed  in 
practically  the  same  time  from  the  effects  of  both  the  exploded  and 
unexploded  gas. 

Autopsies  were  performed  by  Dr.  G.  H.  Hart  on  a  number  of  the 
squirrels  which  had  succumbed  to  the  exploded  gas  and  on  others 
killed  by  the  unexploded  mixture.  The  animals  killed  by  the  unex- 
ploded gas  did  not  show  any  characteristic  lesions.  Their  appearance 
was  in  all  ways  similar  to  that  of  an  animal  which  had  been  killed 
by  any  anaesthetic  such  as  ether  or  chloroform.  The  squirrels  killed 
by  the  exploded  gas  exhibited  the  pink  coloration  of  the  lungs  which 
brightens  on  exposure  to  the  air  and  which  is  commonly  caused  by 
sulfur  dioxide  reacting  with  the  coloring  matter  of  the  blood.  There 
was  no  definite  general  bright  coloration  of  the  body  cavity  character- 
istic of  carbon-monoxide  poisoning,  but  samples  of  the  blood  examined 
by  the  delicate  spectroscope  test  clearly  showed  the  presence  of  carbon 
monoxide,  indicating  that  this  gas  had  contributed  to  the  death  of 
the  squirrels. 

In  order  to  determine  what  portion  of  the  effect  of  the  exploded 
gas  was  due  to  the  sulfur  dioxide,  two  squirrels  were  treated  with  a 
constant  current  of  this  gas  in  the  glass  bell-jar  previously  described. 
The  first  squirrel  was  kept  in  *4%  sulfur  dioxide  gas  which  is  five 
times  as  strong  as  the  Selby  Smelter  Commission  has  found  unsup- 
portable  by  human  beings.     At  the  close  of  an  hour  the  squirrel  was 


214  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

entirely  unharmed  and  did  not  appear  greatly  inconvenienced  at  any 
time.  A  second  animal  was  then  placed  in  2%  sulfur  dioxide  and 
after  thirty-seven  minutes  exposure  succumbed.  It  was  not  overcome 
by  the  gas  at  an  early  stage  of  the  treatment,  as  is  the  case  with  carbon 
bisulfide,  either  exploded  or  unexploded.  In  fact,  it  was  able  to  move 
about  till  just  before  its  death  and  had  it  been  removed  earlier  would 
probably  have  recovered.  Sulfur  dioxide  is  apparently  about  half  as 
deadly  as  carbon  bisulfide  of  the  same  concentration,  but  twice  as 
much  sulfur  dioxide  is  formed  by  the  explosion.  Considering  the 
sulfur  dioxide  alone  we  might  expect  that  the  exploded  and  unex- 
ploded gases  would  be  equally  poisonous,  but  if  sulfur  dioxide  were 
the  only  poisonous  substance  present  the  squirrel  would  be  overcome 
very  slowly.  Since  the  squirrel  is  overcome  and  collapses  in  prac- 
tically the  same  time  with  the  exploded  mixture,  we  infer  that  the 
small  amount  of  carbon  monoxide  formed  contributes  materially  to  the 
effects  causing  the  death  of  the  animals.  The  only  point  which  was 
noticed  that  indicated  that  the  exploded  gas  might  be  any  more  effec- 
tive than  the  unexploded,  was  the  action  upon  fleas.  The  fleas  found 
upon  the  squirrels  killed  in  the  exploded  carbon  bisulfide  were  all 
dead,  while  those  on  the  squirrels  killed  in  the  unexploded  gas  revived 
soon  after  they  were  brought  into  the  open  air. 

FLOW  OF  GAS  UPHILL 

The  next  point  studied  with  the  artificial  burrow  was  the  rate  of 
flow  of  gas  around  corners  and  up-hill.  This  is  a  matter  of  great 
importance  in  the  efficiency  of  practical  operations  in  the  field.  It 
is  generally  difficult  to  say  which  direction  a  burrow  takes  under- 
ground. It  will  therefore  be  impossible  to  tell  whether  the  carbon 
bisulfide,  applied  with  a  waste  ball,  is  placed  at  the  highest  point 
of  a  burrow  or  whether  the  farther  runways  rise  to  a  higher  elevation. 
If  the  gas  cannot  rise  over  elevations  of  one  to  two  feet  it  is  extremely 
doubtful  whether  the  fumigation  will  be  effective. 

The  soil  pipe  was  now  extended  to  four  lengths  and  two  right  angle 
bends  placed  in  it.  The  bottle  with  the  squirrel  was  inserted  in  the 
upper  end  of  the  pipe  and  the  carbon  bisulfide  added  through  the 
opening  in  the  lower  end.  The  difference  in  elevation  was  slightly 
over  eighteen  inches  and  the  distance  between  the  two  ends  was 
twenty-two  and  a  half  feet. 

The  first  squirrel  was  fumigated  with  2%  carbon  bisulfide.  Ap- 
proximately thirty  minutes  elapsed  before  the  squirrel  appeared  to 
detect  any  odor  of  the  gas  and  a  treatment  of  twenty  minutes  more 
was  entirely  without  effect,  the  squirrel  being  apparently  uninjured. 


CONTROL  OF  GROUND  SQUIRRELS  BY  THE  FUMIGATION  METHOD       2\.) 

A  second  squirrel  was  treated  with  2%  carbon  bisulfide  and  the  gas 
exploded.  The  explosion  was  not  vigorous,  because  the  heavy  vapor 
had  not  spread  sufficiently  far  up  the  pipe.  At  the  end  of  an  hour 
the  squirrel  was  still  unhurt.  The  lowest  point  of  the  pipe  was  now 
raised  about  six  inches  so  that  the  vajor  could  spread  through  this 
length  and  2%  gas  was  again  exploded.  The  fumes  of  the  explosion 
reached  the  squirrel  but  not  in  sufficient  quantity  to  affect  it  seriously 
in  an  hour's  treatment.  The  pipe  was  again  lowered  so  that  there  was 
eighteen  inches  difference  in  level  between  the  two  ends.  A  squirrel 
was  placed  at  the  upper  end  and  sufficient  carbon  bisulfide  added  to 
make  a  strength  of  4%  gas  in  the  air.  After  half  an  hour's  treatment 
this  squirrel  began  to  show  signs  of  partial  collapse  but  at  the  end  of 
an  hour  could  still  move  about  briskly.  It  was  taken  out  at  this  time, 
but  died  later  in  the  night.  Another  animal  was  treated  with  4% 
gas  which  was  exploded  and  at  the  close  of  an  hour  was  not  seriously 
affected.  A  large  strong  squirrel  was  now  placed  in  the  upper  end 
of  the  pipe  and  two  ounces  of  carbon  bisulfide  was  added  to  an  inex- 
pensive pump  machine  which  vaporized  the  carbon  bisulfide  and  drove 
the  gas  with  moderate  force  into  the  burrow.  In  ten  minutes  after  the 
pumping  began  the  squirrel  commenced  to  be  affected.  Pumping 
was  continued  at  infrequent  intervals  for  ten  minutes  more.  In 
thirty-five  minutes  from  the  time  the  treatment  started  the  squrrel 
was  dead. 

The  conclusion  to  be  drawn  from  these  experiments  is  that  carbon 
bisulfide  gas  will  flow  readily  on  the  level  or  down-hill.  It  will  not 
in  either  its  exploded  or  unexploded  form  rise  over  an  elevation  of 
even  one  to  two  feet  unless  driven  by  some  form  of  pumping  or  com- 
pressed-air apparatus.  This  undoubtedly  explains  many  failures 
under  field  conditions. 

At  the  close  of  the  carbon-bisulfide  fumigations  two  squirrels,  a 
large  strong  male  and  a  female,  were  treated  in  the  artificial  burrow, 
with  2%  Kilmol.  The  gas  was  allowed  to  flow  from  the  upper  end 
just  as  the  2%  carbon  bissulfide  had  and  the  squirrels  died  in  twenty 
and  twenty-one  minutes,  respectively,  i.e.,  in  the  same  time  as  did 
those  treated  with  carbon  bisulfide. 

A  number  of  queries  have  been  received  as  to  the  possibility  of 
using  other  gases  than  carbon  bisulfide  for  the  purpose  of  exterminat- 
ing squirrels.  These  were  the  result  of  the  accounts  which  had  been 
published  of  the  use  of  gas  in  modern  warfare,  though  occasional 
attempts  to  try  other  fumigants  have  been  made  in  the  past.  A  great 
variety  of  gases  are  known  to  have  been  employed  on  the  European 
battle  front.     The  preparation  of  many  of  these  is  extremely  com- 


216 


PNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


TABLE  II. 
EXPERIMENTS  WITH  OTHER  GASES. 


Percentage 

Manner  of  use 

Sex 

Weight 

Time  of 

Result 

of  gas 

Treatment 

Approx.  4% 

Gasoline   volatilized   incom- 

gasoline 

pletely  in  carboys 

male 

652  g. 

43  min. 

recovered 

Approx.  4% 

Gasoline   volatilized   incom- 

gasoline 

pletely  in  carboys 

female 

435  g. 

52  min. 

death 

Approx.  2% 

Gasoline   volatilized   incom- 

gasoline 

pletely  in  carboys 

male 

652  g. 

1  hr. 

unharmed 

Approx.  2% 

More  completely  vaporized 

gasoline 

in  simple  carburetor 

male 

652  g. 

1  hr. 

*unharmed 

Approx.  4% 

More  completely  vaporized 

gasoline 

in  simple  carburetor 

male 

652  g. 

50  min. 

death 

Approx.  4% 

Exploded    in    artificial    bur- 

gasoline 

row 

female 

large 

25  min. 

unharmed 

2%  Kilmol 

Unexploded,  caged,  artificial 

burrow 

male 

large 

20  min. 

death 

2%  Kilmol 

Unexploded,  caged,  artificial 

burrow 

female 

405  g. 

21  min. 

death 

Approx.  2% 

Caged,  artificial  burrow 

male 

large 

42  min. 

death 

carbon  tetra- 

chloride 

Chlorine 

Caged,  artificial  burrow 

female 

624  g. 

30  min. 

death 

Sulfur 

Undecomposed 

male 

555  g. 

2  hr.  15  min 

unhurt 

Chloride 

Sulfur 

Decomposed  with  water 

male 

555  g. 

3  hrs. 

partial 

Chloride 

recovery 

M%  sulfur 

Constant  stream  of  gas 

female 

large 

1  hr. 

unhurt 

dioxide 

2%  sulfur 

Constant  stream  of  gas 

female 

large 

37  min. 

death 

dioxide 

Arsine-calcium 

arsenide 

2  g.          Small  jar 

male 

large 

25  min. 

death 

15  g.          Caged,  artificial  burrow 

male 

701  g. 

50  min. 

death 

7 . 5  g.          Caged,  artificial  burrow 

female 

535  g. 

45  min. 

death 

7.5  g.          Caged,  artificial  burrow 

female 

470  g. 

38  min. 

death 

5.0  g.          Caged,  artificial  burrow 

female 

492  g. 

46  min. 

death 

7 . 5  g.          Caged,  artificial  burrow 

female 

414  g. 

15  min. 

death  after 

2  hrs. 

7.5  g.          Caged,  artificial  burrow 

female 

501  g. 

10  min. 

death  55  min. 

later 

7 . 5  g.          Caged,  artificial  burrow 

male 

164  g. 

10  min. 

death    after 

several  hrs. 

(I  day  intervened  after  previous  experiment) 


CONTROL  OF  GROUND  SQUIRRELS  BY  THE  FUMIGATION   METHOD       217 

plicated  and  expensive,  and  great  danger  would  attend  their  use. 
Chlorine  is  probably  the  most  readily  available  of  those  which  can 
be  employed.  One  great  difficulty  in  regard  to  the  use  of  chlorine  is 
that  the  liquefied  gas  is  at  present  put  up  only  in  large  cylinders,  in 
most  cases  containing  100  pounds,  though  a  few  eastern  firms  make 
25-pound  cylinders.  The  price  recently  quoted  was  25  cents  per 
pound,  exclusive  of  the  cost  of  the  container. 

As  a  preliminary  experiment  one  squirrel  was  treated  with 
chlorine  generated  on  a  small  scale  in  the  laboratory.  The  effect  of 
the  gas  was  evidently  extremely  painful  and  irritating,  very  differ- 
ent from  the  carbon  bisulfide  which  may  be  slightly  exciting  or  intoxi- 
cating at  first,  but  later  acts  like  an  anaesthetic.  The  squirrel  also 
survived  for  half  an  hour,  a  distinctly  longer  time  than  with  carbon 
bisulfide,  and  made  vigorous  efforts  to  escape.  These  efforts  would 
doubtless  result  in  thejreeing  of  many  animals  under  field  conditions. 
Chlorine  is  an  extremely  corrosive  irritating  gas.  Its  use  would  in- 
volve great  danger  to  the  operators  as  it  readily  attacks  the  mucous 
membrane  of  the  throat  and  nose.  The  use  of  a  gas  under  pressure 
from  small  cylinders  would  have  several  advantages  as  it  would  take 
the  place  of  a  pump  machine,  but  it  is  not  believed  that  this  would 
be  less  expensive  than  carbon  bisulfide. 

Gasoline  has  had  a  number  of  advocates  and  has  on  a  number  of 
occasions  been  used  in  local  campaigns.  A  mixture  was  accordingly 
made  up  in  the  glass  carboy  system  which  would  have  represented 
4%  of  gas  in  the  air  if  it  had  been  completely  volatilized.  One  great 
difficulty  was  observed  with  both  gasoline  and  distillate,  in  that  a  con- 
siderable portion  of  the  material  does  not  evaporate  and  is  therefore 
wasted.  One  squirrel  was  treated  with  this  approximately  4%  gas 
till  it  partially  collapsed,  which  required  43  minutes.  It  was  then 
taken  out  and  next  morning  was  completely  recovered.  A  second 
animal  was  treated  with  the  same  gas  till  it  ceased  to  breathe.  This 
required  52  minutes,  and  it  failed  to  revive. 

An  attempt  was  then  made  to  devise  a  simple  type  of  vaporizing 
carburetor,  but  even  with  this  some  gasoline  remained  as  a  liquid. 
A  squirrel  was  treated  in  gas  from  this  vaporizer  and  expired  in  50 
minutes.  Two  squirrels  were  then  treated  with  approximately  2% 
gasoline  vapor,  one  with  vapor  from  the  liquid  evaporated  spontane- 
ously in  carboys  and  the  other  with  vapor  from  the  simple  carburetor 
and  in  each  case  after  an  hour's  exposure  the  squirrel  was  entirely 
unharmed.  As  a  final  test  a  mixture  of  approximately  4%  gasoline 
vapor  was  made  up  in  the  artificial  burrow,  a  squirred  was  placed  at 
the  lower  end  and  the  gas  was  exploded.     The  squirrel  was  greatly 


218  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

frightened  but  was  entirely  unharmed  either  by  the  explosion  or  by 
the  gaseous  products  formed. 

These  experiments  clearly  show  that  gasoline  is  much  less  effective 
than  carbon  bisulfide  and  even  with  gasoline  at  20  cents  a  gallon  and 
carbon  bisulfide  at  ninety,  the  carbon  bisulfide  would  be  the  less 
expensive  material. 

Carbon  tetrachloride  suggested  itself  because  its  vapor  is  heavy 
and  non-inflammable.  A  squirrel  treated  with  it  died  in  42  minutes. 
The  effect  was  much  the  same  as  with  carbon  bisulfide  except  that  it 
was  much  less  rapid.  It  is  also  a  more  expensive  material,  so  that 
these  two  considerations  would  rule  it  out. 

Sulfur  dioxide,  formed  by  burning  sulfur  has  been  used  occasion- 
ally, but  the  results  of  the  experiments  heretofore  reported  do  not 
warrant  its  application. 

Sulfur  chloride,  a  heavy  unpleasant  liquid  formed  as  a  by-product 
in  the  manufacture  of  chlorine,  has  been  found  very  poisonous  and 
unpleasant  for  workmen  around  chemical  plants.  The  liquid  itself 
is  partially  volatile  and  is  also  decomposed  by  water  into  hydrochloric 
acid  and  sulfur  dioxide.  Two  squirrels  were  treated  with  sulfur 
chloride,  the  first  with  the  undecomposed  liquid.  After  two  hours  it 
was  entirely  unhurt.  The  other  was  subjected  to  the  fumes  formed 
from  the  decomposition  of  the  sulfur  chloride  and  water  and  in  three 
hours'  time  was  only  partially  overcome.  The  material  was  abandoned 
as  valueless. 

Dr.  L.  H.  Duschak  has  suggested  calcium  arsenide  as  a  possible 
fumigant.  This  substance  when  treated  with  water  yields  arsine,  an 
extremely  deadly  gas.  A  sample  containing  50%  calcium  arsenide 
was  supplied  by  the  Bureau  of  Mines  and  a  series  of  squirrels  treated 
with  it  in  the  artificial  burrow.  Amounts  of  five  to  fifteen  grams, 
approximately  one-sixth  to  one-half  ounce,  were  used.  These  amounts 
corresponded  to  3.5%  to  10%  of  arsine  in  the  air  of  the  pipe.  The 
calcium  arsenide  was  placed  in  a  small  dish  in  the  upper  end  of  the 
burrow  and  an  ounce  of  water  was  run  into  the  dish  through  a  glass 
tube.  The  decomposition  of  the  calcium  arsenide  was  practically 
instantaneous  and  the  gas  rapidly  flowed  to  the  lower  end  of  the  pipe. 
The  arsine,  unlike  carbon  bisulfide,  is  an  active  poison  and  not  an 
anaesthetic.  It  does  not,  however,  appear  to  attract  the  attention  of 
the  squirrel  or  cause  any  pain  or  discomfort  until  it  produces  a  more 
or  less  active  nausea.  Where  the  squirrels  were  left  in  the  gas  con- 
tinuously they  died  in  thirty-five  to  fifty  minutes.  This  is  not,  of 
course,  so  rapid  in  its  action  as  is  carbon  bisulfide,  but  it  was  found 
that  if  squirrels  were  left  in  the  gas  for  from  ten  to  fifteen  minutes 


CONTROL  OF  GROUND  SQUIRRELS  BY  THE  FUMIGATION  METHOD       219 

and  then  removed  to  pure  air,  they  still  died  in  from  one  to  two  hours 
afterwards.  This  suggests  that  the  calcium  arsenide  may  possibly 
be  very  effective  in  the  field,  for  even  if  a  squirrel  only  came  in  con- 
tact with  the  gas  for  a  brief  period  of  time  and  then  escaped  it  would 
be  very  likely  to  succumb  later.  The  calcium  arsenide  is  the  only 
new  material  which  appears  to  be  especially  promising,  but  it  is  not 
at  present  on  the  market  and  will  require  careful  trial  before  it  can 
be  recommended  for  general  use. 

FIELD  FUMIGATION  EXPERIMENTS 
At  the  beginning  of  the  investigation  it  was  found  that  very  little 
exact  information  existed  as  to  actual  field  conditions  under  which 
the  fumigation  has  to  be  carried  out.  That  is  to  say,  nothing  definite 
was  known  as  to  the  size,  depth,  or  extent  of  the  burrows  of  the  Cali- 
fornia ground  squirrel.  Various  guesses  were  current  that  the  depth 
of  the  burrow  would  range  from  two  to  six  or  more  feet,  and  the  area 
covered  was  equally  indefinite.  Shaw  of  the  "Washington  Agricultural 
Experiment  Station  has  published  a  series  of  plans  of  the  burrows  of 
the  Columbian  ground  squirrel,7  and  Frandsen  of  Nevada  similar 
drawings  of  the  Oregon  ground  squirrel,8  but  no  such  work  had 
appeared  on  the  California  squirrel. 

A  series  of  preliminary  studies  were  accordingly  made  in  the 
Berkeley  hills  in  collaboration  with  Mr.  Joseph  Dixon  of  the  Museum 
of  Vertebrate  Zoology,  University  of  California.  This  work  was  later 
greatly  extended  by  Dixon  in  the  San  Joaquin  Valley,  and  observa- 
tions on  the  Oregon  ground  squirrel  were  made  by  Grinnell  and 
Jacobsen  in  Siskiyou  County.  Detailed  maps  of  portions  of  this 
work  will  be  published  elsewhere  by  Grinnel  and  Dixon,  but  the 
essential  facts  derived  from  it  are  given  in  Table  III. 


TABLE  III. 

CUBIC  CONTENTS  OF  SQUIRREL  BURROWS 

Length 

Diameter 

Sex 

Cu.  Contents 
including  chambers 

Result  of 
Fumigation 

Berkeley  Hills 

5  ft. 

3.5  in. 

male 

1 .  03  cu.  ft. 

killed  squirrel 

8  ft. 

4.0  in. 

male 

1.40  cu.  ft. 

killed  squirrel 

14  ft. 

4.5  in. 

male 

2 .  40  cu.  ft. 

killed  squirrel 

22  ft. 

4.5  in. 

female 

4.8  cu.  ft. 

killed  squirrel 

Bakersfield 

34  ft. 

4.5  in. 

male 

4.8  cu.  ft. 

killed  squirrel 

(vicinity) 

138  ft. 

4.75  in. 

Colony 

17.78  cu.  ft. 

squirrel  escaped 

Fresno  Area 

26  ft. 

5.0  in. 

male 

4.6  cu.  ft. 

squirrel  escaped 

Oregon  Ground 

Squirrel 

Siskiyou  County 

66  ft. 

2.5  in. 

female 

3.48  cu.  ft. 

no  fumigation 

220  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

The  great  variation  in  length  and  consequently  in  cubic  content 
is  immediately  evident.  In  the  Berkeley  hills  the  male  burrows  were 
found  on  the  outer  edge  of  the  ground  occupied  by  large  colonies. 
These  burrows  were  in  several  cases  quite  short,  though  in  others  they 
extended  to  a  length  of  fourteen  or  fifteen  feet.  The  greatest  depth 
reached  by  the  male  burrows  in  this  region  was  eighteen  inches  to  two 
feet,  while  the  complete  female  burrow  examined,  reached  a  length  of 
twenty-two  feet  and  an  extreme  depth  of  thirty  inches.  Some  of  these 
burrows  were  treated  by  the  waste  ball  method,  both  exploded  and 
unexploded,  and  some  by  pumping.  In  all  of  them  the  animals  were 
found  killed  near  the  nest.  The  ground  was  level  or  gently  sloping 
and  therefore  presented  no  difficulties  of  treatment.  Under  such  con- 
ditions any  method  of  treatment  by  fumigation  would  be  effective. 

The  burrows  treated  by  Dixon  in  the  Bakersfield  and  Fresno  areas 
of  the  San  Joaquin  Valley  were  in  each  case  dosed  by  the  waste  ball 
method.  The  first  one,  in  the  vicinity  of  Bakersfield,  was  a  compara- 
tively simple  male  burrow  thirty-four  feet  long.  One  entrance  to  the 
burrow  was  closed  and  in  digging  the  squirrel  was  found  some  twenty 
feet  from  the  entrance.  On  digging  further  it  was  found  that  there 
was  another  entrance  at  the  other  end  of  the  thirty-four  feet.  This 
had  been  unobserved  and  was  left  open,  but  the  squirrel  was  neverthe- 
less killed.  The  ground  was  almost  level  and  here  again  the  gas  had 
a  perfect  chance  to  flow.  In  the  other  burrow  no  squirrels  were  killed 
because  it  was  a  large  colony  burrow  of  greater  length  and  with 
greater  cubical  air  contents. 

The  burrow  dug  out  in  the  Fresno  area  was  located  in  the  "hog 
wallow"  country  and  is  an  excellent  illustration  of  the  extremely 
difficult  conditions  which  may  be  encountered.  The  land  has  many 
layers  of  hardpan  which  are  not  continuous  and  between  these  the 
squirrel's  burrow  twists  and  turns.  The  side  view  of  this  burrow  is 
reproduced  here  (figure  3)  as  it  shows  so  clearly  the  problem  involved. 
The  burrow  in  question  was  treated  by  the  waste  ball  method.  It 
was  then  dug  out  and  the  discovery  was  made  that  the  gas  had  not 
risen  over  "c";  the  squirrel  being  found  alive  in  the  nest.  This 
points  to  the  necessity  of  using  some  form  of  apparatus  to  pump  the 
gas.  The  depths  of  the  burrows  in  the  valley  areas  were  found  to  be 
greater  than  those  in  the  Berkeley  hills  where  the  shale  subsoil  tends 
to  limit  the  habited  zone.  The  greatest  depth  reached  in  the  San 
Joaquin  Valley  was  four  to  five  feet,  which  is  believed  to  be  typical 
of  conditions  in  the  interior  valleys. 

The  Oregon  ground  squirrel  is  only  of  importance  in  the  extreme 
northern  section  of  the  state,  but  where  found  is  exceptionally  numer- 


CONTROL  OF  GROUND  SQUIRRELS  BY  THE  FUMIGATION   METHOD       221 


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222  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

ous  and  prolific.  It  is  a  distinctly  smaller  species,  as  is  indicated  by 
the  diameter  of  the  burrows,  which  are  in  addition  tortuous  and 
winding. 

CONCLUSIONS 

The  series  of  experiments  here  described  clearly  shows  that  carbon 
bisulfide  is  an  eminently  satisfactory  fumigant  for  ground  squirrels. 
Twenty  minutes'  to  a  half -hour's  treatment  with  air  containing  2% 
of  carbon  bisulfide  is  certain  to  be  fatal.  The  concentration  of  gas 
actually  used  in  practice  will  of  necessity  vary  with  the  volume  of  the 
burrow  which  of  course  is  not  known  to  the  operator.  The  dosage  of 
iy2  to  2  ounces  usual  in  the  waste  ball  method  corresponds  to  about 
2%  gas  in  what  we  consider  to  be  large  burrows.  The  concentration 
in  small  burrows  would  be  proportionately  greater.  Unless  a  burrow 
happens  to  be  much  larger  than  any  we  have  found,  the  concentration 
produced  with  the  usual  dosage  should  be  sufficient  to  kill  in  about 
20  minutes,  always,  provided  the  vapor  is  completely  diffused. 

It  is  not  suffiicient  under  adverse  conditions  to  merely  drop  the 
carbon  bisulfide  in  the  burrow  on  a  waste  ball.  This  will  produce  good 
results  if  everything  is  favorable.  When  the  ground  is  level  or  slightly 
sloping  the  gas  will  flow  for  twenty  or  thirty  feet  to  the  lowest  part 
of  the  burrow.  Our  experiments  with  the  artificial  burrow,  as  well 
as  Dixon's  field  observations  in  the  San  Joaquin  Valley,  demonstrate 
that  the  gas  will  not  flow  over  elevations  of  one  or  two  feet.  Explod- 
ing the  gas  will  distribute  it  somewhat  more  but  not  sufficiently  to 
make  it  rise  over  the  usual  irregularities. 

This  clearly  indicates  that  some  form  of  apparatus  to  pump  the 
gas  is  desirable.  The  commercial  machines  which  were  formerly 
on  the  market  were  all  too  expensive,  ranging  in  price  from  five  to 
twenty-five  dollars.  It  is  clear  that  all  that  is  necessary  is  a  bellows 
or  large-bored  pump  which  will  blow  a  strong  stream  of  air  through 
the  tank  which  contains  liquid  carbon  bisulfide.  For  field  work  in  the 
San  Joaquin  or  Sacramento  Valley,  where  it  is  possible  to .  drive 
readily  over  the  country,  a  blacksmith's  rotary  blower,  or  hand  spray 
outfit  could  be  connected  to  a  vaporizing  tank  and  mounted  on  a 
wagon  with  one  man  to  operate  the  pump  and  another  to  fumigate. 
In  this  way  great  efficiency  may  be  obtained.  If  a  power  spray  outfit 
were  adapted  for  the  purpose  it  would  be  necessary  to  observe  great 
care  to  keep  carbon  bisulfide  gas  away  from  the  exhaust  of  the  engine 
and  avoid  the  possibility  of  a  serious  explosion.  It  is  believed  that  any 
handy  person  ought  to  be  able  to  devise  a  satisfactory  fumigation 
apparatus  out  of  materials  to  be  found  on  the  average  ranch.    Circular 


CONTROL  OF  GROUND  SQUIRRELS  BY  THE  FUMIGATION  METHOD       223 

181  of  this  station  illustrates  and  gives  dimensions  of  the  complete 
squirrel  "Destructor"  which  could  be  simplified  in  many  ways. 

The  only  difference  in  field  procedure  which  is  advisable  when 
carbon  bisulfide  is  used  in  a  pump  machine,  is  that  the  refined 
instead  of  the  crude  carbon  bisulfide  be  used.  This  will  necessitate 
ordering  in  large  quantity  from  the  wholesaler  as  only  the  crude 
article  is  usually  handled  by  the  local  dealers.  The  price  is  ordinarily 
about  the  same,  but  the  refined  product  will  be  found  much  more 
desirable  for  pump  machines  as  the  crude  article  ordinarily  contains 
a  small  amount  of  free  acid  which  will  corrode  the  vaporizing  tank. 

It  will  be  found  necessary  to  vary  the  dose  of  carbon  bisulfide 
employed  depending  on  the  nature  of  the  country  in  which  the  squir- 
rels are  located.  In  foothill  sections  of  shallow  soil  and  hard  subsoil, 
similar  to  the  Berkeley  hills,  treatments  which  average  half  an  ounce 
to  a  hole  will  be  sufficient  if  a  pump  machine  is  employed.  Where 
deep  valley  soils  prevail  aud  especially  where  large  colony  burrows  are 
found  it  will  be  necessary  to  use  two  or  three  times  as  much  carbon 
bisulfide  to  secure  satisfactory  results. 

The  laboratory  investigations,  as  well  as  the  field  observations  of 
Dixon,  show  that  neither  gasoline  nor  distillate  are  satisfactory  mate- 
rials for  squirrel  fumigation.  Their  effectiveness  is  so  much  lower 
that,  as  previously  stated,  even  if  20  cents  per  gallon  is  paid  for 
gasoline  and  90  cents  for  carbon  bisulfide  the  bisulfide  will  be  found 
less  expensive  per  squirrel  killed.  It  is  believed  that  the  field  has 
been  thoroughly  canvassed  for  possible  fumigating  materials,  and  with 
the  exception  of  calcium  arsenide,  which  will  require  further  work  to 
prove  its  possibilities,  carbon  bisulfide  is  the  most  satisfactory  chemical 
so  far  developed. 

Intelligent  observation  and  careful  work  will  be  found  to  yield  good 
results  at  every  stage  of  a  squirrel  campaign.  In  the  use  of  fumigating 
materials  it  is  possible  to  effect  great  economy  by  treating  only  oc- 
cupied burrows.  The  poisoning  campaign  which  precedes  the  fumi- 
gation should  have  been  arranged  for  the  earliest  possible  date  that 
will  find  the  squirrels  actively  working  in  the  spring.  A  week  or 
ten  days  after  the  poison  has  been  distributed  a  workman  equipped 
with  a  mattock  should  go  over  the  infested  areas  and  close  every 
visible  hole.  The  fumigators  should  follow  in  one  to  two  days.  Every 
hole  that  is  now  open  will  indicate  that  the  burrow  is  occupied  by  a 
squirrel  and  very  few  empty  holes  will  be  treated  by  this  method. 
The  treated  holes  will  be  again  closed  so  a  later  inspection  should 
clearly  indicate  whether  any  squirrels  have  escaped. 

It  is  too  much  to  expect  complete  eradication  of  all  squirrels  in 


224  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

any  one  or  two  seasons'  work.  California  is  too  widely  infested  with 
ground  squirrels  for  such  a  thing  to  be  possible.  A  few  squirrels  will 
escape  in  gullies  or  dry  washes.  Even  under  the  best  possible  condi- 
tions we  must  face  the  necessity  of  squirrel  control  just  as  the  orchard- 
ist  faces  an  annual  outlay  for  spraying  or  fumigation.  The  encourag- 
ing thing  about  squirrel  control  is  that  the  work  will  become  less  and 
less  if  it  is  intelligently  performed  and  thus  pay  larger  and  larger 
returns  on  the  time  and  materials  invested. 

BIBLIOGEAPHY 

i  Merriam,  C.  H.,  California  Ground  Squirrels.  Treasury  Dept.  Public  Health 
and  Marine  Hospital  Service,  1909,  pp.  1-8. 

Dixon,  Joseph,  Univ.  of  Calif.  Agr.  Exp.  Sta.  Circular  181,  pp.  1-14. 

Nelson,  E.  W.,  Smaller  Mammals  of  North  America,  National  Geographic 
Magazine,  Vol  XXXIII,  No.  5,  May,  1918,  pp.  371-493. 

2  Hilgard,  E.  W.,  Lecture  on  the  Phylloxera  or  Grape  Vine  Louse.  Bull. 
Univ.  of  Calif.  No.  23,  January,  1876,  pp.  1-24. 

3  Hilgard,  E.  W.,  Destruction  of  the  Ground  Squirrel  by  the  Use  of  Bi-sulfide 
of  Carbon.     Bull,  of  Univ.  of  Calif.,  April,  1878,  No.  32,  pp.  1-6. 

4  Long,  J.  D.,  A  Squirrel  Destructor.  Treasury  Dept.  U.  S.  Public  Health 
Service,  Eeprint  from  Public  Health  Eeports,  No.  98,  1912,  pp.  1-9. 

5  Stewart,  G.  R.  and  Burd,  John  S.,  Course  of  Reaction  in  Explosions  of 
Dilute  CS    Air  Mixtures.     Journal  of  Indus.  &  Eng.  Chem.   (In  press). 

6  Holmes,  J.  A.,  Franklin,  C.  C,  and  Gould,  Ralph  A.,  Report  of  the  Selby 
Smelter  Commission.     Dept.  of  the  Interior,  Bur.  of  Mines,  Bull.  98,  pp.  1-528. 

7  Shaw,  W.  T.,  Ground  Squirrel  Control.  Washington  Station,  Popular  Bull. 
99,  1916,  pp.  11,  pi.  2,  figs.  4. 

8  Frandsen,  P.,  Ground  Squirrels  and  other  Rodent  Pests  in  Nevada.  Sta. 
Bull.  58,  pp.  34,  pi.  5,  figs.  7,  March,  1905. 


